1. Technical Field
The invention concerns a piston ring.
2. Related Art
Piston rings are provided with wear-protection coatings on the bearing surface and/or on the ring sides to be able to meet the required service life. Higher cylinder pressures, direct fuel injection, exhaust gas return, and other structural features of new engine development, such as alternative cylinder materials as well as minimizing oil consumption, are increasingly burdening piston rings.
Wear-protection coatings are applied by means of thermal spray processes, galvanic methods, or thin-layer technology and, if need be, are treated by heat treatment and diffusion processes. The coatings are as a rule largely homogeneous and thus are applied unstructured. The wear resistance is adjusted by means of matching hardness of the material.
All manifestations that indicate thermal overstressing at the piston-ring surface are typically lumped together under the term of “burn traces”.
The term “burn traces” was originally defined by manifestations on the running surface of piston rings. Discoloration and so-called “cobblestone formation” on the chromed surface are evidence of thermal overload. If the term “burn traces” is also extended to other materials, then all manifestations that refer to thermal overstressing on the piston-ring surfaces are to be included in it. The differences between burn traces and scoring are not clear. Severe grooving and/or material transfer on the surface of piston rings is known as scoring. Burn traces and scoring are caused by metal contact of the co-operating component, which occurs due to a lack of lubrication oil or too high a pressure per unit area on the piston rings. Leaking piston rings not adjacent to the cylinder wall and out-of-round cylinders facilitate the undesirable passage of hot fuel gases (blow-by loss), whereby the lubrication oil is burned up and metal contact of the co-operating component occurs (see www.motorlexikon,de).
A galvanic hard-chrome coating is known from DE 199 31 829 A1, in which diamond particles with a size of 0.25 to 0.5 μm are embedded. Additionally, then, even hard particles of material made up of tungsten carbide, chromium carbide, aluminum oxide, silicon carbide, silicon nitride, boron carbide, or boron nitride can be embedded in the cracks.
At higher temperatures, the diamond particles are converted to graphite, which then assumes the task of lubrication task and thereby prevents the formation of burn traces. Consequently, this coating also has very good emergency running-mode properties, particularly due to the conversion of diamond to graphite at temperatures of about 700° C. or higher.
In order to further improve the burn-trace behavior of piston rings, coatings up to now have typically been used made out of materials that have very high melting points and for the thermal overstressing of which, as a result of high temperatures, they are required. A typical example of this is chromium nitride applied by means of a physical vapor deposition (PVD) process with a decomposition temperature of approximately 2000° K.
In order to improve the resistance to burn traces and wear resistance, a coating is proposed in DE 10 2004 028 486 A1 of several individual layers, which consists alternately of chromium and chromium nitride. The chromium nitride layers can be of CrN, Cr2N, or mixtures thereof. To avoid a sudden transition, the coating process is controlled so that the individual chromium-nitride layers exhibit on both sides an edge of Cr2N and a core of CrN. Each individual layer is at least 0.01 μm thick. The maximum thickness is 10 μm. The total thickness of the coating offered is 5 to 100 μm.
The U.S. Pat. No. 5,549,086 discloses piston-ring coatings of TiN and CrN.
The German patent DE 10 2004 032 403 B3 describes piston rings which exhibit at a chromium adhesive layer a CrN gradient layer with a nitrogen content increasing outward.
Piston rings for internal combustion engines are known from the Japanese patent JP 2005-060810 A which are provided with a multilayer coating system, the individual layers of which themselves exhibit metal components and differ merely in nitrogen content. Coating thicknesses of the individual layers are offered at <1 μm. The coatings are applied by means of a PVD process, especially an electric arc process.
The burn-trace resistance of known coatings is not satisfactory however.